Picture display cell with memory effect

Claim 1

An image display cell with a memory effect comprising a housing having a transparent wall part and containing a reversible reduceable redox material dissolved in an electro-chemically inert liquid which is in contact with at least two inert electrodes provided in said housing, characterized in that the electrochemically inert liquid is a mixture of water and an electrochemically inert solvent in which the redox material is a 4,4'-bipyridinium compound corresponding to the formula I ##STR11## wherein R 1 and R 2 each represent an alkyl group having 4-8 carbon atoms and X - is a ClO 4 ', NO 3 ', H 2 PO 4 ' or BF 4 ' group, or a bipyridinium dimeric compound which corresponds to the formula II ##STR12## wherein R 3 and R 4 each represent an alkyl group having 1-4 carbon atoms or a benzyl group which may be substituted with an electrochemically inert substituent, and Y - is a ClO 4 ', BF 4 ', H 2 PO 4 ' or PF 6 ' group, or a bipyridinium polymeric compound which is obtained by reacting 4,4'-bipyridyl and α,ω-dibromobutane in a polar aprotic solvent and then replacing the bromide anion by a ClO 4 ' ion, H 2 PO 4 ' ion or BF 4 ' ion the combination of said mixture of water and solvent and redox material being such that said redox material when in reduced form is colored and insoluble in said mixture of water and solvent.